This invention relates to an independent dispersive power source which employs a solar battery or the like as an electric source and which feeds electric power to a load through an inverter, and more particularly to a power source device which can be switched to the feed of electric power from a power supply system as a stand-by electric source in the case of insufficient sunshine or any abnormality.
FIG. 1 is a block diagram showing a prior-art power source device of this type. Referring to the figure, numeral 1 designates a solar battery, numeral 2 an accumulator which is disposed in parallel with the output of the solar battery 1, numeral 3 an inverter which changes the D.C. power of the constituent 1 or 2 into an alternating current of predetermined frequency and voltage, and numeral 4 an output transformer. An A.C. switch 5a turns `on` or `off` the output from the inverter 3, while an A.C. switch 5b turns `on` or `off` electric power from a system to be described below. Numeral 6 indicates a load, and numeral 7 the power distribution system (stand-by power supply system). A control circuit 8 for the inverter 3 is constructed of a reference oscillator 10 of quartz or the like which determines the frequency of the inverter 3, a reference voltage source 11 which generates the output voltage reference of the inverter 3, means 12 to detect the output voltage of the inverter 3, an amplifier 13 which amplifies the deviation between the voltages of the reference voltage source 11 and the output voltage detection means 12, a PWM circuit 14 which generates a signal determinative of the pulse width of the inverter 3 in accordance with the amplified voltage deviation signal, and a gate circuit 15 which amplifies the signal from the PWM circuit 14 and gives drive signals to the frequency switches of the inverter 3.
Next, the operation of the power source device will be described. D.C. power generated in the solar battery 1 is changed into the A.C. power of predetermined frequency and voltage in the inverter 3 under the action of the control circuit 8, and the A.C. power is fed to the load 6 through the output transformer 4 as well as the A.C. switch 5a. In this case, the switch 5b for the feed from the system 7 is open, and a complete independent power source loop is established. As regards the inverter 3, a train of oscillating pulses from the oscillator 10 to determine the output frequency and the amplified signal of the deviation between the output voltage reference signal of the reference voltage source 11 and the output voltage value from the detection means 12 are input to the PWM circuit 14, a train of pulses of pulse widths corresponding to the inputs are output therefrom and are impressed on the gate circuit 15, and the outputs of this gate circuit are distributed to the corresponding semiconductor switches of the inverter 3 so as to drive them, with the result that the output voltage and the frequency of the inverter are held constant. In this manner, the inverter shown here is arranged and operated as is well known.
Meanwhile, the difference between the quantity of the power generation of the solar battery 1 and the quantity of the power consumption of the load 6 is compensated by the charge and discharge of the accumulator 2, to keep both the quantities balanced. However, in a case where insufficient sunshine has continued, the balance cannot be maintained, and the remaining power quantity of the accumulator 2 decreases gradually. Such a case is detected with, e.g., the remaining power quantity of the accumulator 2 as a criterion by an operator or sensing means not shown. Also, a trouble having arisen in the inverter by way of example is detected by sensing means. On this occasion, the operator or the sensing means opens the A.C. switch 5a and thereafter closes the A.C. switch 5b on the system side, thereby to switch the power source device to the feed of power from the system 7. To the contrary, when the sunshine energy has been recovered or the trouble has been eliminated, the feed of power to the load is switched from the system side to the inverter side. At this time, the A.C. switch 5b is first opened, and the A.C. switch 5a is thereafter closed. Subsequently, the inverter needs to feed the load with power after, if necessary, it is started with the so-called soft start of gradually increasing a voltage in order to suppress the inrush currents of a transformer etc. included in the load.
In the prior-art power source device thus far described, the conventional inverter control circuit which is simple is used. On the other hand, however, the momentary interruption of feed power is inevitable at the time of the switching of the power feed to the load, not only in a case of emergency such as accident, but also in a case of the resumption of normal conditions or the planned switching of power feed required for the operation of the device. When mechanical switches are employed, the period of the interruption amounts to several tens cycles, so that influence on the load appears. Moreover, the problems of turn-on in the opposite phase, etc. arise in a motor load etc. In case of switching the power feed from the system to the inverter, therefore, the aforementioned soft start or the like needs to be performed with a sufficient period for the purpose of preventing the inrush overload of the inverter.
In order to shorten the period of the starting mode, semiconductor switches such as thyristors need to be employed as the A.C. switches 5a and 5b, and the whole device cannot but become complicated and expensive. Even in this case, the momentary service interruption cannot be prevented.
Apart from the independent power source device, there is a device similar thereto which is normally connected in parallel with a distribution system and in which only when an inverter has gotten out of order, it is separated. Also the device of this type involves such problems that the connection with the system incurs influences from the inverter upon the system, such as higher harmonics, reverse voltage and reverse current, and that when the service of the system has interrupted, also the inverter stops. The reason is that, unlike the case of the independent power source device in FIG. 1, the inverter is controlled following up the voltage and frequency of the system received as reference inputs.